std::ranges::search_n
来自cppreference.com
<tbody>
</tbody>
<tbody class="t-dcl-rev t-dcl-rev-num ">
</tbody><tbody>
</tbody>
<tbody class="t-dcl-rev t-dcl-rev-num ">
</tbody><tbody>
</tbody>
| 在标头 <algorithm> 定义
|
||
| 调用签名 |
||
| (1) | ||
template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Pred = ranges::equal_to, class Proj = std::identity > requires std::indirectly_comparable<I, const T*, Pred, Proj> constexpr ranges::subrange<I> search_n( I first, S last, std::iter_difference_t<I> count, const T& value, Pred pred = {}, Proj proj = {} ); |
(C++20 起) (C++26 前) |
|
template< std::forward_iterator I, std::sentinel_for<I> S, class Pred = ranges::equal_to, class Proj = std::identity, class T = std::projected_value_t<I, Proj> > requires std::indirectly_comparable<I, const T*, Pred, Proj> constexpr ranges::subrange<I> search_n( I first, S last, std::iter_difference_t<I> count, const T& value, Pred pred = {}, Proj proj = {} ); |
(C++26 起) | |
| (2) | ||
template< ranges::forward_range R, class T, class Pred = ranges::equal_to, class Proj = std::identity > requires std::indirectly_comparable <ranges::iterator_t<R>, const T*, Pred, Proj> constexpr ranges::borrowed_subrange_t<R> search_n( R&& r, ranges::range_difference_t<R> count, const T& value, Pred pred = {}, Proj proj = {} ); |
(C++20 起) (C++26 前) |
|
template< ranges::forward_range R, class Pred = ranges::equal_to, class Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj> > requires std::indirectly_comparable <ranges::iterator_t<R>, const T*, Pred, Proj> constexpr ranges::borrowed_subrange_t<R> search_n( R&& r, ranges::range_difference_t<R> count, const T& value, Pred pred = {}, Proj proj = {} ); |
(C++26 起) | |
1) 在范围
[first, last) 中搜索首个 count 个元素的连续序列，其投影后的值按照二元谓词 pred 均等于给定的值 value。2) 同 (1)，但以
r 为源范围，如同以 ranges::begin(r) 为 first 并以 ranges::end(r) 为 last。此页面上描述的函数式实体是算法函数对象（非正式地称为 niebloid），即：
参数
| first, last | - | 要检验的（又称草堆）元素范围的迭代器-哨位对 |
| r | - | 要检验的元素范围（又称草堆） |
| count | - | 要搜索的序列长度 |
| value | - | 要搜索的值（又称针） |
| pred | - | 比较投影后元素与 value 的二元谓词
|
| proj | - | 应用到要检验的范围的元素的投影 |
返回值
1) 返回含有范围
[first, last) 中指代找到的序列的一对迭代器的 std::ranges::subrange。
若找不到这种序列，则返回 std::ranges::subrange{last, last}。
count ≤ 0 则返回 std::ranges::subrange{first, first}。2) 同 (1)，但返回类型为
ranges::borrowed_subrange_t<R>。复杂度
线性：至多应用 ranges::distance(first, last) 次谓词与投影。
注解
若迭代器实现了 std::random_access_iterator 则实现可以提升平均搜索效率。
| 功能特性测试宏 | 值 | 标准 | 功能特性 |
|---|---|---|---|
__cpp_lib_algorithm_default_value_type |
202403 |
(C++26) | 算法中的列表初始化 |
可能的实现
struct search_n_fn
{
template<std::forward_iterator I, std::sentinel_for<I> S,
class Pred = ranges::equal_to, class Proj = std::identity,
class T = std::projected_value_t<I, Proj>>
requires std::indirectly_comparable<I, const T*, Pred, Proj>
constexpr ranges::subrange<I>
operator()(I first, S last, std::iter_difference_t<I> count,
const T& value, Pred pred = {}, Proj proj = {}) const
{
if (count <= 0)
return {first, first};
for (; first != last; ++first)
if (std::invoke(pred, std::invoke(proj, *first), value))
{
I start = first;
std::iter_difference_t<I> n{1};
for (;;)
{
if (n++ == count)
return {start, std::next(first)}; // 找到
if (++first == last)
return {first, first}; // 找不到
if (!std::invoke(pred, std::invoke(proj, *first), value))
break; // 不等于 value
}
}
return {first, first};
}
template<ranges::forward_range R,
class Pred = ranges::equal_to, class Proj = std::identity,
class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
requires std::indirectly_comparable<ranges::iterator_t<R>, const T*, Pred, Proj>
constexpr ranges::borrowed_subrange_t<R>
operator()(R&& r, ranges::range_difference_t<R> count,
const T& value, Pred pred = {}, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r),
std::move(count), value,
std::move(pred), std::move(proj));
}
};
inline constexpr search_n_fn search_n {};
|
示例
运行此代码
#include <algorithm>
#include <cassert>
#include <complex>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>
int main()
{
namespace ranges = std::ranges;
static constexpr auto nums = {1, 2, 2, 3, 4, 1, 2, 2, 2, 1};
constexpr int count{3};
constexpr int value{2};
typedef int count_t, value_t;
constexpr auto result1 = ranges::search_n
(
nums.begin(), nums.end(), count, value
);
static_assert // 找到
(
result1.size() == count &&
std::distance(nums.begin(), result1.begin()) == 6 &&
std::distance(nums.begin(), result1.end()) == 9
);
constexpr auto result2 = ranges::search_n(nums, count, value);
static_assert // 找到
(
result2.size() == count &&
std::distance(nums.begin(), result2.begin()) == 6 &&
std::distance(nums.begin(), result2.end()) == 9
);
constexpr auto result3 = ranges::search_n(nums, count, value_t{5});
static_assert // 找不到
(
result3.size() == 0 &&
result3.begin() == result3.end() &&
result3.end() == nums.end()
);
constexpr auto result4 = ranges::search_n(nums, count_t{0}, value_t{1});
static_assert // 找不到
(
result4.size() == 0 &&
result4.begin() == result4.end() &&
result4.end() == nums.begin()
);
constexpr char symbol{'B'};
auto to_ascii = [](const int z) -> char { return 'A' + z - 1; };
auto is_equ = [](const char x, const char y) { return x == y; };
std::cout << "找到了子序列 " << std::string(count, symbol) << " 于 ";
std::ranges::transform(nums, std::ostream_iterator<char>(std::cout, ""), to_ascii);
std::cout << '\n';
auto result5 = ranges::search_n(nums, count, symbol, is_equ, to_ascii);
if (not result5.empty())
std::cout << "找到其位置在 "
<< ranges::distance(nums.begin(), result5.begin()) << '\n';
std::vector<std::complex<double>> nums2{{4, 2}, {4, 2}, {1, 3}};
#ifdef __cpp_lib_algorithm_default_value_type
auto it = ranges::search_n(nums2, 2, {4, 2});
#else
auto it = ranges::search_n(nums2, 2, std::complex<double>{4, 2});
#endif
assert(it.size() == 2);
}
输出：
找到了子序列 BBB 于 ABBCDABBBA
找到其位置在 6
参阅
(C++20) |
查找首对相同（或满足给定谓词）的相邻元素 (算法函数对象) |
(C++20)(C++20)(C++20) |
查找首个满足特定条件的元素 (算法函数对象) |
(C++20) |
查找元素序列在特定范围中最后一次出现 (算法函数对象) |
(C++20) |
搜索一组元素中任一元素 (算法函数对象) |
(C++20) |
当一个序列是另一个的子序列时返回 true (算法函数对象) |
(C++20) |
查找两个范围的首个不同之处 (算法函数对象) |
(C++20) |
搜索元素范围的首次出现 (算法函数对象) |
| 搜索元素在范围中首次连续若干次出现 (函数模板) |